1. Field of the Invention
The present invention relates to television receivers and, more particularly, to television receivers having a picture-in-picture (PIP) feature, that is receivers in which a small, compressed version of a video signal is displayed within a larger "main" picture.
2. Description of the Prior Art
In television receivers having a PIP feature, two channels are provided, namely a main channel and a PIP channel. The main channel is a conventional television channel, receiving the incoming composite television signal and furnishing a luminance and two color difference signals. The PIP channel also receives the composite television signal, but it is processed digitally. A digital, reduced version of the received picture is stored in a memory. It is read out from memory, the read-out data is converted back to analog form, and the resulting color difference and luminance signals are combined with the corresponding signals from the main channel under control of a synchronous switching circuit. The combined signals are then converted to the red, blue and green signals required to control the display.
A vertical prefilter is provided to process the digital television signal before it is stored in memory. Since the picture is to be displayed in a much smaller area than the main picture, some sampling is required. However, sampling at less than the Nyquist frequency introduces low frequency beats. Also, the compressed signal, if not preprocessed, may be subject to line flicker and aliasing.
A filter is therefore provided which processes the output of the analog/digital converter. The filter is essentially a lowpass filter which reduces the high vertical frequencies in the signal before it is subsampled in a PIP processor connected in cascade with the filter. The PIP processor accepts only every third line for storage in the PIP memory.